In recent years, in order to promote recycling of waste materials, various types of waste materials are increasingly used as a cement raw material or burning fuel. Because some of the waste materials used as the cement raw material or the burning fuel, such as ash from incinerated general municipal wastes, various types of sludge, coal ash, displaced soil at construction, or the like, contain the heavy metal, there is concern that the amount of the heavy metal brought into the cement production system increases.
Among the heavy metals brought into a cement production process, mercury, zinc, selenium, these chlorides, etc. are volatilized at the high temperature sections (for example, a rotary kiln, a pre-heater, etc.) of the cement production system, and are contained in the exhaust gas. Then, in connection with the temperature of the exhaust gas falling, these heavy metals are deposited on the surface of the dust contained in the exhaust gas, or become the fine particles of the heavy metals and the compounds themselves.
These dust and particles are collected by a dust collector (an electrostatic precipitator, a bag filter, etc.) disposed in the exhaust gas channel of the cement production system, and removed out of the exhaust gas.
When the dust or the like collected by this means is reused as a part of the cement raw material and the burning fuel, most of the volatile heavy metals contained in the dust or the like are volatilized again at the high temperature sections of the cement production system and led again into the exhaust gas channel in a condition where it is contained in the exhaust gas.
Since the mercury among such heavy metals has high volatility and is easy to gasify in high temperature, it is hardly contained in the clinker, but is contained in the exhaust gas. Thus a part of the mercury is exhausted out of the system with the exhaust gas, but most of the mercury is to be circulated in the exhaust gas system. For this reason, if a means for removing the mercury in the exhaust gas is not equipped, the concentration of mercury in the exhaust gas becomes high gradually in accordance with the amount of mercury brought into from the cement raw material or the like increasing, and the problem that the mercury exhausted to the outside of the system increases arises.
Therefore, in order to reduce the amount of mercury in the exhaust gas in the cement production system, a method has been proposed in which the dust contained in the exhaust gas exhausted from the cement burning equipment is collected by the electrostatic precipitator or the like, the dust is then led to the heating furnace, the volatile metallic component contained in the dust is removed by heating the dust to the temperature equal to or more than the volatilization temperature of the volatile metallic component and gasifying the volatile metallic component, and the dust from which the volatile metallic component is removed is used as part of the cement raw material (refer to Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-open No. 2002-355531